"...the first step in the revolution by the working class is to raise the proletariat to the position of ruling class to win the battle of democracy. The proletariat will use its political supremacy to wrest, by degree, all capital from the bourgeoisie, to centralise all instruments of production in the hands of the State, i.e., of the proletariat organised as the ruling class; and to increase the total productive forces as rapidly as possible." — Marx and Engels, The Communist Manifesto
"When, in the course of development, class distinctions have disappeared, and all production has been concentrated in the hands of a vast association of the whole nation, the public power will lose its political character. Political power, properly so called, is merely the organised power of one class for oppressing another. If the proletariat during its contest with the bourgeoisie is compelled, by the force of circumstances, to organise itself as a class, if, by means of a revolution, it makes itself the ruling class, and, as such, sweeps away by force the old conditions of production, then it will, along with these conditions, have swept away the conditions for the existence of class antagonisms and of classes generally, and will thereby have abolished its own supremacy as a class. In place of the old bourgeois society, with its classes and class antagonisms, we shall have an association, in which the free development of each is the condition for the free development of all." — Marx and Engels, The Communist Manifesto
The important clause there is "If the proletariat during its contest with the bourgeoisie is compelled, by the force of circumstances, to organise itself as a class, if, by means of a revolution..." That's not a call for revolution. That's saying that if and only if the bourgeoisie prevents the working class from gaining its power through the democratic process, revolution will be necessary.
ETA: The other important clause: "...to wrest, by degree, all capital..." Marx was an incrementalist.